The present invention relates to the use of solutions of zinc salts of fatty acids in aliphatic and optionally aromatic polyamines as additives for producing a compact surface in the production of polyurethane based molded foams using water as the blowing agent.
The production of molded polyurethane foam products having a compact surface by foaming inside a mold is already known in the art (see, e.g., German Auslegeschrift 1,196,864). The known process is carried out by introducing a reactive and foamable mixture of organic polyisocyanates, compounds having isocyanate reactive groups and the usual auxiliary agents and additives into a mold in a larger quantity than would be required for filling the mold under conditions of unrestricted foaming, and then foaming up the mixture inside the mold. Both flexible and semi-rigid as well as rigid molded articles can thus be produced by suitable choice of the starting components, in particular by suitable choice of their molecular weights and functionality. The dense outer skin is obtained by introducing a larger quantity of foamable mixture into the mold than would be required for filling the volume of the mold under conditions of unrestricted foaming and by using fluorochlorohydrocarbons as blowing agents. These blowing agents condense on the internal wall of the mold under the prevailing temperature and pressure conditions so that the blowing reaction comes to a standstill on the internal wall of the mold and a compact outer skin is formed.
Apart from the above-mentioned physical blowing agents, water is used as chemical blowing agent in industrial polyurethane chemistry in the form of the carbon dioxide produced by the reaction between water and isocyanates. Although this chemical blowing agent can be used to produce polyurethane foams of excellent quality under conditions of unrestricted foaming, it is incapable of giving rise to high quality molded foams having a compact surface (integral foams). This is due to the fact that the carbon dioxide does not condense against the internal wall of the mold under the usual conditions and the blowing action is therefore not stopped in the skin zone.
The use of zinc salts as internal mold release agents for polyurethane elastomers is known in the art (see, e.g., published European patent application 119,471, German Offenlegungsschrift 3,639,502, and U.S. Pat. Nos. 4,519,965, 4,581,386, 4,766,172 and 4,895,879).